Storage Management Subsystem (SMS).
It is basically where for a particular dataset u provide storage class.
For SMS-managed data sets, by the storage class for the new data set,
specified on the STORCLAS parameter of the DD statement or selected by the installation-written Automatic Class Selection (ACS) routine for the new data set.
For an SMS-managed data set, SMS obtains information about the device to be used for the data set based on the storage class assigned for the data set.
The device used by the storage class that an ACS routine selects is sufficient for the data sets you create with DD statements.
You can, however, specify the name of a storage class on the STORCLAS
parameter for a new SMS-managed data set. (Note that an ACS routine can override the storage class that you specify.)
To let an ACS routine select a storage class for a new data set, omit the
//DD5 DD DSNAME=CHECK.PGM,DISP=(NEW,KEEP)
To specify a specific storage class for a new data set, code the STORCLAS
//DD6 DD DSNAME=CHECK.PGM,STORCLAS=STOR55,DISP=(NEW,KEEP)